The identification of essential effector genes in lupus has provided important information both in terms of the molecules, mechanisms and pathways involved in the disease process and in identifying possible therapeutic targets. Currently, however, identification of such genes requires substantial breeding, is relatively inefficient, and is limited by both the availability of knockout genes and the requirement that the function of the gene be known. Recently, the forward genetics approach using ENU mutagenesis has been applied successfully in both large and small projects to dissect specific molecular pathways involved in normal and disease states. Although application of this strategy to spontaneous models of SLE is precluded by a number of factors (including the requirement that crosses, to map ENU mutations, must have homogeneous disease severity), published and preliminary data provide strong support for using the mercury-induced autoimmunity model (HgIA) to identify essential effector genes relevant to spontaneous SLE. This approach has the potential for identifying most of the major genes required for the development of HgIA, of which many will be applicable to SLE, and furthermore, in contrast to the current methods, it does not require prior knowledge about the function of the mutated genes in autoimmunity. By potentially opening up the entire genomic repertoire for screening, this proposed project is likely to identify new classes of genes and novel mechanisms. To apply the ENU approach to define essential effector genes in HgIA and SLE, two specific aims are proposed. Aim 1 will generate ENU mutants that are resistant to the systemic autoimmunity induced by mercuric chloride. We propose to screen a total of 800 ENU-mutated gametes, which would cover ~12,700 recessive loss-of-function mutations equivalent to about half the genome. Aim 2 will map and identify the specific ENU-induced genetic alteration in selected mutants. The results of the proposed studies should define many of the early components and mechanisms required for the production of autoantibodies in SLE and will likely identify novel genes and pathways that could be targeted for therapeutic intervention.